The present invention relates to a stepping motor, and particularly a stepping motor with a disc-shaped multipolar magnet.
A motor of this type is described in the American patent U.S. 4,330,727. The motor according to this patent comprises a rotor having a traversing motor axis and a disc-shaped, multipolar magnet fixed on the axis by intermediate retainers. The rotor is mounted into a stator and supported by two bearings, one on each side of the disc magnet, in the form of ball bearings. This motor has two phases and comprises two coils surrounding magnetic circuit elements in each stator half. The magnetic circuits have air gaps within which the rotor magnet is positioned.
In a motor of this type, the drive organ for the object to be displaced is fixed on the motor axis outside the housing. In certain applications, the overall dimensions of such an assembly relative to the torque delivered and the number of steps per revolution are unsatisfactory. On the other hand, one always seeks to reduce the manufacturing costs, improve the precision, enhance the performance, and in particular, to increase the speed and the torque.
It is an object of this invention to realise a stepping motor that is compact, precise, and efficient. It is advantageous for certain applications to have a motor that is short in the direction of the axis of rotation. It is also advantageous to reduce the manufacturing costs of stepping motors.
The objects of this invention are achieved by a stepping motor according to claim 1.
In the present invention, a stepping motor comprises a stator with at least two magnetic circuits and a rotor that is rotatably mounted on a stator via bearings, the rotor comprising an organ equipped with magnets or a multipolar magnet essentially disc-shaped that is mounted on a portion of the rotor, said portion of the rotor being fixed to a radially external portion of the magnet and a radially internal portion of the magnet being arranged within the air gap of said magnetic circuits.
The portion of the rotor can have a central opening through which a portion of the stator extends.
The design according to the invention advantageously allows a particularly compact stepping motor to be realised.
The rotor portion can be in the shape of an essentially flat ring. The rotor portion can further comprise teeth arranged on its periphery in order to engage complementary teeth or holes of an object or organ to be displaced.
The rotor portion and/or magnet can be fixed on a mobile ring of the bearing, preferably the external ring of the bearing, the bearing preferably being a ball bearing. The ball bearing can be a bearing withstanding axial and radial stresses. The internal ring of the bearing can be mounted on a cylindrical surface of a portion of the stator. The internal ring of the bearing can be shorter than the external ring of the bearing against which the rotor portion is set, in order to reduce the length of the motor in the direction along its axis of rotation.
The design according to the invention allows a motor with a single bearing to be realised, notably a ball bearing with four contact points and large diameter, which results in an extremely compact yet highly rigid design. Moreover, the motor consists of a small number of pieces, so that its manufacture is economical.
The stator may comprise a first stator portion on which the coils are mounted, and a second stator portion in the form of yokes set against this first portion so as to close the magnetic circuits which comprise an air gap between the radially outer portions of the first stator portion and the yokes, respectively. There is one yoke for each motor phase, so that the magnetic circuits of the different phases are separate. Each magnetic circuit can thus comprise a coil and a yoke.
For a two-phase motor, two coils and two yokes are symmetrically arranged on either side of a line of symmetry passing through the center of rotation of the motor. The radially outer portions of the yokes and of the first stator portion can be fitted with teeth or other extension shapes facing each other to form stator pole pairs attracting the opposite pole pairs of the magnet when the coil of the corresponding magnetic circuit is supplied with electric current. The pole pairs of the magnet move into a position facing the stator pole pairs, whereupon the other coil is supplied so as to attract magnet pole pairs, and so on, to make the rotor move forward. The teeth or extensions of each yoke portion are symmetrically arranged with respect to the line of symmetry but are not diametrically symmetric, since the teeth, as in the conventional stepping motors, move forward by angles in such a way that the magnet poles facing extensions or teeth of one magnetic circuit do not face teeth or extensions of the other magnetic circuit.
It should be noted that the motor according to the invention can have more than two phases without leaving the scope of the invention. For example, the motor can have several magnetic circuits taking up motor sectors of generally cylindrical shape, a sector having an angle of essentially 120xc2x0 for a three-phase motor and an angle of 90xc2x0 for a four-phase motor.
The first stator portion on which the coils are mounted can be made of a sintered material having a good magnetic permeability but poor electrical conductivity, so as to limit eddy current losses in a compact, low-cost design.